The present application is based on Japanese priority application No.2002-056146 filed on Mar. 1, 2002, the entire contents of which are hereby incorporated by reference.
The present invention generally relates to semiconductor devices and more particularly to the method of forming a ferroelectric film and fabrication process of a semiconductor device having a ferroelectric film.
A ferroelectric random access memory (FRAM) is a semiconductor device that has a ferroelectric capacitor. A ferroelectric capacitor is a capacitor having a ferroelectric film sandwiched by a pair of capacitor electrodes, and a ferroelectric random access memory stores information in the ferroelectric capacitor insulation film in the form of spontaneous polarization. Thus, an ferroelectric random access memory functions as a non-volatile memory device.
In view of the fact that reversal of spontaneous polarization can be caused in a ferroelectric capacitor insulation film by mere application of a reversing electric field thereto without injection of a current, a ferroelectric random access memory can be written with information (reversal of spontaneous polarization) with minimum electric power. Further, such reversal of the spontaneous polarization can be conducted with very high speed.
Typically, a ferroelectric capacitor for use in a ferroelectric random access memory uses a layer of a perovskite ferroelectric material such as PZT (Pb(Zr,Ti)O3), SBT(SrBi2Ta2O9), and the like, for the ferroelectric capacitor insulation film, and the ferroelectric capacitor insulation film is formed on a lower electrode formed of a material such as Pt (platinum). The lower electrode, in turn, is connected to a conductive region of a transistor by way of a conductive plug such as W(tungsten) provided in an interlayer insulation film.
In order that the ferroelectric film undergoes the desired reversal of polarization with minimum drive voltage, it is necessary that the crystal grains in the ferroelectric film are aligned in a predetermined crystal orientation. Typically, a crystal orientation of  less than 111 greater than  or  less than 001 greater than  is used in the case of a PZT film, wherein the crystal orientation of  less than 001 greater than  is most preferable in view of the maximum electric performance of the film.
On the other hand, there is a possibility in such a conventional construction that the tungsten plug or platinum lower electrode undergoes oxidation at the time of formation of the ferroelectric capacitor insulation film because of the oxidizing ambient that is used at the time of formation of the ferroelectric film.
In view of the problems noted above, the inventor of the present invention has proposed the use of Ir(iridium) for the material of the electrode of the ferroelectric capacitor used in a ferroelectric random access memory. In the case of using iridium, the problem of increase of resistance at the lower capacitor electrode caused by oxidation of the lower capacitor electrode is minimized because of the fact that iridium oxide.
When using iridium for the lower electrode film, it is preferable to form the ferroelectric film by way of an MOCVD process in view of the controllability of the ferroelectric composition and in view of feasibility of mass production.
At the time of forming a ferroelectric film by way of an MOCVD process, it is necessary to cause a decomposition of metal organic source materials of the ferroelectric film in a reactor such that deposition of a ferroelectric film takes place on the iridium substrate, wherein it should be noted that such a decomposition process requires oxygen.
On the other hand, oxygen thus introduced into a reactor, while causing the necessary decomposition of the metal organic source materials of the ferroelectric film, causes also an unwanted oxidizing reaction on the surface of the iridium film, on which the deposition of the ferroelectric film takes place. As a result of such an oxidizing reaction, the surface of the iridium lower electrode is converted to iridium oxide (IrO2).
When the surface of the iridium lower electrode is completely oxidized and there is formed a film of iridium oxide covering the surface of the Ir electrode, the crystal grains of the ferroelectric film thus grown on the iridium oxide film take various crystal orientations, and the desired  less than 001 greater than  orientation is no longer obtained. Thereby, problems such as increase of drive voltage at the time of writing or erasing of information are caused associated with the degradation of the electric performance of the ferroelectric film, which in turn is caused by the uncontrolled alignment of orientation of the crystal grains in the ferroelectric film.
Further, it has been difficult to control the progress of the oxidization reaction of the iridium lower electrode and it has been difficult to control the timing of ferroelectric film formation such that the formation of the ferroelectric film is started before there is caused substantial oxidation in the iridium lower electrode.
Accordingly, it is a general object of the present invention to provide a novel and useful method of forming a ferroelectric film and fabrication process of a semiconductor device having a ferroelectric film wherein the foregoing problems are eliminated.
Another object of the present invention is to provide a method of forming a ferroelectric film while controlling the atmosphere of the ferroelectric film formation at a critical point of oxidation-reduction reaction.
Another object of the present invention is to provide a method of forming a ferroelectric film, comprising the steps of:
forming a layer by a material that takes a metal state in a reducing ambient and an oxide state in an oxidizing ambient; and
depositing a ferroelectric film on a surface of said layer by supplying gaseous sources of said ferroelectric film and an oxygen gas and causing a decomposition of said gaseous sources at said surface of said layer,
said step of depositing said ferroelectric film being started with a preparation step in which the state of said surface of said layer is controlled substantially to a critical point in which the state of said layer changes from said metal state to said oxide state and from said oxide state to said metal state.
Another object of the present invention is to provide a method of fabricating a semiconductor device, comprising the steps of:
forming an active element on a substrate;
forming a lower electrode over said substrate in electrical connection with said active element, said step of forming said lower electrode being conducted by using a material that takes a metal state in a reducing ambient and an oxide state in an oxidizing ambient for said lower electrode;
depositing a ferroelectric film on a surface of said lower electrode by supplying gaseous sources of said ferroelectric film and an oxygen gas and by causing a decomposition of said gaseous sources at said surface of said lower electrode; and
depositing an upper electrode on said ferroelectric film,
said step of depositing said ferroelectric film being started with a preparation step in which the state of said surface of said lower electrode is controlled substantially to a critical point in which a state of said lower electrode changes from said metal state to said oxide state and from said oxide state to said metal state.
According to the present invention, the surface of the layer is only partially covered by an oxide film such that the crystal structure of the material forming the layer is exposed at the time of starting the deposition of the ferroelectric film as a result of the preparation step. Alternatively, the surface of the layer is covered with an extremely thin oxide film such as the one having the thickness of several atomic layers of oxygen or less, at the time of starting the deposition of the ferroelectric film by a CVD process. As a result, the ferroelectric film deposited on the layer inherits the crystal orientation of the layer in the metallic state even when the deposition of the ferroelectric film is conducted under the existence of the oxygen gas, and the ferroelectric film thus grown shows excellent electric performance.
In a preferred embodiment, a preparation step is conducted after the formation of the layer but before the start of deposition of the ferroelectric film, by processing the surface of the layer by an organic solvent. Thereby, layer causes a catalytic action enhancing the reducing power of the organic solvent, and the surface state of the layer is effectively biased to the reducing side at the time the deposition of the ferroelectric film is started and the oxygen gas is supplied together with other various metal organic gaseous sources, and there appears an equilibrium state in which the reducing reaction and the oxidizing reaction are equilibrated. Once the growth of the ferroelectric film thus is started with the predetermined desired crystal orientation determined with the crystal orientation of the layer, the crystals in the ferroelectric film grow in conformity with the ferroelectric crystal grains at the interface to the layer, and the entire ferroelectric film is grown in the desired crystal orientation.
Another object of the present invention is to provide a method of realizing a critical state of oxidation-reduction reaction on a surface of a layer.
Another object of the present invention is to provide a method of realizing a critical state of oxidation-reduction reaction on the surface of a layer that takes a metal state in a reducing ambient and an oxide state in an oxidizing ambient and shows a catalytic action in the metal state, comprising the steps of:
introducing said layer into a process space;
supplying an oxidizing gas and a vapor of an organic solvent into said process space; and
causing an oxidizing reaction and a reducing reaction on said surface of said layer in said process space,
said oxidizing gas and said vapor being supplied into said process space with a proportion set such that there is caused an equilibrium between said oxidizing gas and said vapor on said surface of said layer.
Another object of the present invention is to provide a method of determining the state of an oxidation-reduction reaction taking place on a layer of a material that takes a metal state in a reducing ambient and an oxide state in an oxidizing ambient, said material showing a catalytic action in said metal state, said method comprising the steps of:
introducing said layer into a process space;
supplying an oxidizing gas and a vapor of an organic solvent into said process space;
causing an oxidizing reaction and a reducing reaction on said surface of said layer in said process space;
examining a state of said surface of said layer; and
determining a ratio of said oxidizing gas and said reducing gas that provides an equilibrium condition in which an oxidizing reaction and a reducing reaction equilibrate.
Another object of the present invention is to provide an apparatus for forming a ferroelectric film, comprising:
a reactor evacuated by a pump;
a stage provided in said reactor for holding a substrate;
a first source supplying an oxidizing gas to said reactor;
a second source supplying a reducing gas to said reactor;
a third source supplying a gaseous source material of said ferroelectric film to said reactor;
a detector detecting an oxide film formed on said substrate on said stage; and
a controller controlling said first through third sources,
said controller controlling said first and second sources in response to an output of said detector such that there is realized a critical state of oxidation-reduction reaction taking place on a surface of said substrate.
According to the present invention, it becomes possible to determine the specific ratio of the oxidizing gas and the reducing gas in order to achieve the critical condition of the oxidation-reduction reaction taking place on the surface of the layer. By using such a critical condition, it becomes possible to realize the critical condition at the time of depositing a ferroelectric film on the layer to form a ferroelectric capacitor.
Other objects and further features of the present invention will become apparent from the following detailed description when read in conjunction with the attached drawings.